Course of Raku / Advanced / Containers / Ordered containers

Binding with :=

So far, every variable you created spawned a new container, and assignment with = placed a value into that container. Binding, written with :=, is different: it makes a name refer to an existing container instead of creating a new one.

After binding, the two names share the same container, so a change made through one of them is visible through the other:

my $x = 10;
my $y := $x;

$x = 20;
say $y; # 20

Here, $y := $x does not copy the value 10. It makes $y another name for the very same container as $x. When $x is later set to 20, reading $y returns 20 as well.

Compare this with ordinary assignment, which copies the value into a separate container:

my $x = 10;
my $y = $x; # a plain copy

$x = 20;
say $y; # 10

Binding works with arrays too. The following makes @alias another name for @data:

my @data = 1, 2, 3;
my @alias := @data;

@alias[0] = 99;
say @data; # [99 2 3]

One more detail: if you bind a name directly to a literal value, there is no container behind it, so the name becomes read-only:

my $pi := 3.14;
$pi = 3;

Cannot assign to an immutable value
  in block <unit> at t.raku line 2

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